The M21B is significantly larger, and has a sloped head design. The M2, being smaller and with a straight head design, is a lot more pocket- friendly, even with its thicker battery tube, this is why I like it so much. Also, the M21B lacks the SS crenulated bezel of the M2.
For these reasons I feel the M2 is the better design although its thicker battery tube was created before the time of 21700 batteries, and if the inside diameter was just slightly enlarged it would give it a huge advantage over over similar options.